The invention relates to the use of freeze concentration methods and apparatus for separating two or more substances with different freezing points from a solution of a mixture of such substances.
Freeze concentration involves three basic steps. The solution comprising a mixture of substances is cooled until one of the substances freezes out in the form of crystals. The residual solution, called the "mother liquor," is thus concentrated. The crystals are then separated from the concentrated mother liquor. The separated crystals have adhered to their surfaces a quantity of the concentrated mother liquor.
Where the end product is essentially pure crystal, the adhered mother liquor must be removed. Where the end product is the mother liquor, the adhered mother liquor must be removed in order not to sacrifice yield.
Removal of the adhered mother liquor from the crystals is the most critical operation of the three basic steps. The most efficient means for removing mother liquor adhering to a crystal is the wash column, and for this reason, this discussion will be based on the use of a wash column. Other washing devices may be used. For a given volume of slurry being supplied to a wash column, the efficiency of the wash column increases significantly as the size of the crystals increase.
More importantly, larger crystals coated with a less viscous liquor makes it possible to use a smaller and less complicated wash column.
The patent to Ganiaris, U.S. Pat. No. 3,283,522, recognizes and teaches that it is difficult to wash crystals from more viscous concentrated solutions. He also recognizes that additional crystal growth occurs in less concentrated solutions from which they may be more easily washed. Ganiaris' solution, however, is a highly-complicated multi-stage crystallizer network and multiple crystal purifying means.
The principal of concentration gradient columns is described in U.S. Pat. No. 3,681,932 to Huber.
The patent to Shaul, U.S. Pat. No. 3,402,047, describes a system wherein feed is added to crystals after concentrated solution is removed to provide a slurry having a more dilute solution. However, the process of accomplishing this differs considerably from the present invention and presents a greater load on the purifier.